The invention relates to a transformer assembly for stepping up an a.c. power for application to sign lamps (cold cathode discharge tubes such as a neon tube or an argon tube) to light it and provided with protective functions against abnormalities such as non-grounding of a neutral point of a transformer casing, a connection to an a.c. power source in reverse polarities and the like.
FIG. 1 shows a conventional lighting transformer assembly with protective functions. Specifically, a transformer 11 includes a primary winding 12, across which a first and a second input terminal 14, 15 are connected, and a secondary winding 13. It will be noted that a power interrupting switch 16 is connected in series between the first input terminal 14 and one end of the primary winding 12. An a.c. power source such as a commercial power supply 17 is connected across the first and second input terminal 14, 15. The opposite ends of the secondary winding 13 are connected to a first and a second output terminal 18, 19, respectively, across which a discharge tube or tubes 21 such as neon tubes or argon tubes are connected.
The a.c. power from the source 17 is fed to the primary winding 12, which steps it up, thus allowing a high tension a.c. power to be supplied from the secondary winding 13 to the discharge tubes 21 in order to light them.
In the event of occurrence of a ground fault on the secondary side such as a connection of the secondary winding 13 to the ground as a result of a contact of a wiring of the discharge tube 21 with a neon tower, such a ground fault is detected by an abnormality detection circuit 22, the arrangement being such that a detection output is applied to an interrupter circuit 23, which is effective to turn the power interrupting switch 16 off to interrupt the supply of the a.c. power to the primary winding 12, thus preventing a continued current flow through the point of ground fault of the secondary side to cause a fire.
A duty is imposed upon transformer such as the transformer 11 mentioned above to connect a transformer casing 24 to the ground before use. If a person forgets to connect a ground terminal 15 of the casing 14 to the ground and the assembly is put to use, this is detected by the abnormality detection circuit 22 to activate the interrupter circuit 23 to turn the switch 16 off. In a similar manner, if the commercial power supply 17 is connected to the first and the second input terminal 14, 15 in reverse polarities, this is again detected by the abnormality detection circuit 22 to turn the switch 16 off. A protective circuit 20 including the abnormality detection circuit 22 which detects the occurrence of one or more of a variety of abnormalities relating to the transformer 11 and the interrupter circuit 23 which turns the switch 16 off to interrupt the supply of the a.c. power to the transformer 11 in response thereto is contained in the transformer casing 24. The interrupter circuit 23 has the function of maintaining the switch 16 off once it is turned off. By way of example, the switch 16 may comprise relay contacts, and a movable contact of the relay is connected to the first input terminal 14 and is arranged to be switched from a normal closed contact 16NC to a normally open contact 16NO to close a self-holding circuit for the relay. The interrupter circuit 23 is connected across the first and the second input terminal 14, 15 to be fed from the a.c. power applied across the first and the second input terminal 14, 15.
A neon sign may be formed by discharge tubes 21 such as neon tubes or argon tubes, which may be flashed to achieve an advertisement effect. At this end, a flasher 10 is connected between the commercial power supply 17 and the first and the second input terminal 14, 15 to interrupt the supply of the a.c. power to the first and the second input terminal 14, 15 in various forms, causing the discharge tubes 21 to be flashed in various forms as a result of such interruption. A conventional arrangement for the abnormality detection circuit 22 and the interrupter circuit 23 which detect the occurrence of a ground fault and interrupts the supply of the input a.c. power is shown in FIG. 2, designating corresponding parts to those shown in FIG. 1 by like reference numerals. In this example, the secondary winding 13 has a midpoint 41 which is connected to the ground terminal 25. A pair of tertiary windings 13t1, 13t2 which are magnetically coupled to opposite halves located on the both sides of the midpoint 41 of the secondary winding 13 form part of the abnormality detection circuit 22. Normally, the tertiary windings 13t1, 13t2 are juxtaposed on a magnetic core on which the secondary winding 13 is disposed between the lowermost layers thereof such that a layer of insulating material having a high withstand voltage on the order of 6000-7000 V is interposed between the tertiary windings 13t1, 13t2 and the secondary winding 13 to provide a high electrical insulation therebetween while allowing a satisfactory magnetic coupling between the secondary winding 13 and the tertiary windings 13t1, 13t2.
At their one end, the tertiary windings 13t1, 13t2, are connected together in an inverse phase relationship so that their induced voltages cancel each other while at their other end, the tertiary windings 13t1, 13t2 are connected to an input of a rectifying and smoothing circuit 42, the output of which is connected through a Zener diode 46 across a parallel circuit comprising a resistor and a capacitor. The parallel circuit 47 is connected across the gate and the cathode of a triac 30. The triac 30 is connected across the input terminals 14, 15 through a relay drive coil 16c, which when energized, controls relay contacts that define the switch 16.
Under a normal condition, voltages induced across the tertiary windings 13t1, 13t2 are substantially equal in magnitude, but are opposite in phase, whereby an input voltage to the rectifying and smoothing circuit 42 is nearly zero. However, upon a ground fault of the sign lamps 21 or a wiring thereof, one end of the secondary windings which is associated with the ground fault will be short-circuited to the midpoint 41, causing a substantial decrease in the induced voltage in the tertiary winding which is coupled with this secondary winding 13 to allow the full induced voltage across the other tertiary winding to be applied to the rectifying and smoothing circuit 42. This voltage is rectified and smoothed and an increase in the rectified and smoothed output voltage turns the Zener diode 46 on, with consequence that the triac 30 is turned on to energize the relay drive coil 16c to open the switch 16, thus interrupting the supply of the input a.c. power to the transformer 11. The movable contact of the switch 26 comprising the relay contacts is thrown to the normally open position 16NO, whereby the holding current to the relay drive coil 16c flows.
A ground fault protective circuit is shown in FIG. 3, with corresponding parts to those shown in FIG. 2 being designated by like reference characters as used before. Specifically, the midpoint 41 of the secondary winding 13 is connected to the ground terminal 25 through a rectifying diode 37 and a series circuit including a Zener diode 38 and a light emitting element 55PE of a photocoupler 55. The midpoint 41 of the secondary winding is also connected through a resistive element 39 to the ground terminal 25. A series circuit including the relay drive coil 16c and a light receiving element 55PR of the photocoupler 55 is connected across the input terminals 14 and 15. It is to be noted that on the opposite sides of the midpoint 41, the secondary winding 13 is wound in the opposite directions.
Normally, the potentials at the output terminals 18 and 19 alternate between positive and negative maximum values in mutually phase opposition relationship, while the potential at the midpoint 41 remains substantially equal to zero. However, if a ground fault occurs on one of the output terminals, for example, at terminal 18, this output terminal 18 assumes a substantially zero potential, and the potential at the output terminal 19 alternates with respect to the ground with an amplitude which is nearly twice the potential during a normal operation, with consequence that the potential at the midpoint 41 alternates. The resulting potential of the midpoint 41 is rectified by the diode 37 to produce a current flow through the light emitting element 55PE through the Zener diode 38, causing the element 55PE to emit light, which is then received by the light receiving element 55PR to conduct, thus allowing a current flow through the relay coil 16c to cause the contact 16 to be switched from the normally closed position to the normally open position, thus interrupting the supply of the a.c. power to the primary winding 12.
In order to facilitate locating a site where the fault has occurred, the transformer is provided with protective function disable means 27 as shown in FIG. 1. Specifically, if a protective function disable switch 28 is turned on when the protective circuit 20 functions to interrupt the switch 16, the protective function disable circuit 29 is activated to override or invalidate a self-holding circuit, not shown, which is contained in the interrupter circuit 23. For example, in the arrangement of FIG. 2, the series circuit including the movable contact of the switch 16, the normally open contact 16NO, the resistive element 57 and the relay coil 16c is interrupted, and the power interrupting switch 16 is thrown to the normally closed contact 16NC to allow the a.c. power to be supplied to the primary winding 12. When the switch 16 which is once interrupted in response to the ground fault is restored in this manner, there occurs a current flow through the site of ground fault, producing sparks or ozone, which can be relied upon to locate the site of ground fault in a relatively simple manner.
An appearance of sign lamps lighting transformer with protective functions of the kind described is shown in FIG. 4. Specifically, the transformer casing 24 which is rectangular has one end plate on which the first and the second input terminal 14, 15, one output terminal 19, the casing ground terminal 25, and an operating knob 28p of the protective function disable switch 28 are mounted and the other end plate on which the other output terminal 18 is mounted to project therefrom.
As shown in FIG. 5, the protective function disable switch 28 is mounted on the inner surface of the end plate 24a of the transformer casing 24, and the operating knob 28p projects externally through a small opening formed in the end plate 24a. A wiring substrate 35 is disposed within the transformer casing 24 in opposing relationship with the inner surface of the end plate 24a, and while not shown, the protective circuit 20 and the protective function disable circuit 29 are mounted on the substrate, with the protective function disable switch 28 being connected to the protective function disable circuit 29 through a lead wire 36.
It is an object of the present invention to provide a sign lamp lighting transformer assembly with protective functions which is capable of maintaining protective functions if the power supply to a transformer is interrupted by a flasher.
It is another object of the present invention to provide a sign lamp lighting transformer assembly with protective functions which is capable of automatically eliminating a malfunctioning of the protective circuit while allowing a site of ground fault to be located in a facilitated manner.
It is a further object of the present invention to provide a sign lamp lighting transformer assembly with particular functions which facilitates the operation of a protective function disable switch.
It is an additional object of the present invention to provide a sign lamp lighting transformer assembly with protective functions which prevents a malfunctioning for a ground fault and reliably detects a true ground fault.
According to a first aspect of the present invention, a sign lamp lighting transformer assembly with protective functions also comprises a third input terminal, and an interrupter circuit is connected between the third input terminal and one of the input terminals which is not connected to a power interrupter switch, and an a.c. power source is connected across these input terminals so that the interrupter circuit can be fed if the supply of the a.c. power to the primary winding is interrupted.
According to a second aspect of the present invention, a sign lamp lighting transformer assembly with protective functions comprises a restart circuit which is automatically operative whenever a power interrupting switch is turned off by a protective circuit to restore the power interrupting switch to its on condition only once after a brief interval on the order of 0.5 to 1.0 second.
The restart circuit may comprise a drive circuit and a restoring circuit, for example. The drive circuit includes a series capacitor in its current path, and is activated whenever the power interrupting switch is turned off to allow a charging current to flow through the capacitor, and an interrupter circuit is controlled in a manner such that the current flow through the capacitor drives the restoring circuit to turn the power interrupting switch on.
According to a third aspect of the present invention, a sign lamp lighting transformer assembly with protective functions comprises an operating knob for a protective function disable switch which is mounted on a surface of a transformer casing other than the surfaces on which input terminals and/or output terminals are mounted.
According to a fourth aspect of the present invention, a sign lamp lighting transformer assembly with protective functions comprises a rectifier circuit which converts an input a.c. power into a d.c. power, which is then converted into a high frequency high tension power through an inverter and a transformer for application to a sign lamp. A rectifying element and a resistive element are connected in series between the midpoint of a secondary winding of the transformer and a negative output of the rectifier circuit, and a current flow through the resistive element is detected by a detection circuit to be subject to a decision by a decision circuit to see if the detected current has exceeded a given value. If a decision is rendered that the current has exceeded the given value, the operation of the inverter is stopped by a stop circuit in response to the decision.